Police have detained 20 foreign nationals with links to the attack on a popular Istanbul nightclub during New Year celebrations on Sunday in Turkey's western Izmir province.

The suspects, who were questioned with the help of translators, denied having links to the Daesh terror group which was responsible of killing 39 people at the nightclub.

One of the suspects who claimed to be Syrian was found to be from Morocco.

Police are still investigating the nationality of the remaining suspects who were carrying fake passports.

The police intelligence bureau is also trying to identify the suspects with the help of Uzbekistan, Kazakhstan, Morocco, Kirghizstan and some European countries.

At least 35 people had previously been detained as part of the investigation into the attack, according to a police source.

A full investigation into the terror attack remains underway, a source, who spoke on condition of anonymity due to restrictions on talking to the media, said.

At least 39 people, including a police officer, were killed when an attacker opened fire on partygoers at the Reina nightclub in Istanbul's Ortaköy district in the early hours of New Year's Day; 69 other people were also hurt, with a number of foreign nationals among the casualties.